Trimix 10/50 is a gas mixture that contains 10% oxygen and 50% helium, and the rest is nitrogen. if a tank of trimix 10/50 has a total pressure of 2.07 × 10^4 kpa, then what is the partial pressure of helium?

Trimix 10/50 is a gas mixture that contains 10% oxygen and 50% helium, and the rest is nitrogen. If a tank of Trimix 10/50 has a total pressure of 2.07 × 104 kPa, then what is the partial pressure of nitrogen? 
Ptotal = Poxygen + Phelium + Pnitrogen 
2.07 x104 kPa = [2.07x104 x .10] + [2.07x104x .5] + Pnitrogen 

A gas effuses 4.0 times faster than oxygen (O2). What is the molecular mass of the gas? 
O2 is 16g/mol. if a gas effuses 4.0 times faster, then it has to be 4.0 times lighter. divide 16 by 4. 

Above which point on a phase diagram can you no longer distinguish between a liquid and a gas? 
Critical point

A reaction requires 22.4 L of gas at STP. You have 50.0 L of gas at 101.5 kPa and 373 K. Which of the following statements is true? (Use the ideal gas law: PV = nRT where R = 8.31 L-kPa/mol-K.) 
You will have excess gas for the reaction. plug in all the numbers and find n for each reactions.